This invention relates to metallurgical processing, and, more particularly, to the filtration of molten metal and composite materials to remove solid material therefrom.
According to one approach, cast composite materials may be formed by melting a metallic matrix alloy in a furnace and adding particulate matter to the molten metal. The mixture is vigorously mixed to encourage wetting of the matrix alloy to the particles, and after a suitable mixing time the mixture is cast into molds or forms. The mixing is conducted while minimizing the introduction of gas into the mixture. The resulting composite materials have the particulate reinforcement distributed throughout a matrix of an alloy composition.
Such cast composite materials are much less expensive to prepare than many other types of metal-matrix composite materials such as those produced by powder metallurgical technology and infiltration techniques. Composite materials produced by this approach, as described in U.S. Pat. Nos. 4,759,995, 4,786,467, and 5,028,392, have enjoyed commercial success in only a few years after their first introduction.
There are two types of solid matter that may be present in the composite material. A desirable particulate is the ceramic material intentionally added to the melt. This material is usually a carefully selected and sized ceramic. Typical types of ceramics are aluminum oxide and silicon carbide, and typical particle sizes are in the range of from about 5 up to about 35 micrometers. An undesirable solid matter is an uncontrolled material that finds its way into the melt during the production operation. The undesirable solid matter may include, for example, pieces of the ceramic furnace lining that have broken off during mixing, pieces of impellers that have broken off during mixing, pieces of molten-metal furnace troughs that have broken off into the flow metal, pieces of oxide films that have formed on the melt surface and been enfolded into the melt during mixing, and pieces of reaction products between the desirable particulate and the melt that have become free floating in the melt, such as aluminum carbides.
The undesirable solid matter is generally larger in size than the desirable particulate reinforcement, and may typically be on the order of 200 micrometers or more in maximum dimension (i.e., about 10 times the size of the desirable particulates). If left in the melt, the undesirable solid matter is frozen into the composite material when it solidifies. The undesirable solid matter becomes inclusions that can adversely affect the mechanical properties of the final composite material.
A similar problem is encountered in the more-conventional metallurgical industry that does not deal with composite materials. It has long been the practice to filter undesirable solid matter from melts of non-composite alloys that are to be used in sensitive applications. Different types of filters are used, depending upon the metal to be filtered and the cleanliness requirements of the product.
In aluminum alloy melting practice the molten alloy may be passed through a glass-fiber sock filter having an open weave so that there are openings of a predefined size in the filter. The solid matter is trapped at the surface of the filter. The filter openings are typically on the order of 400 micrometers or more in size, and are selected according to the cleanliness requirement of the final product and production considerations. Smaller openings remove smaller particles, resulting in a cleaner final product. On the other hand, the smaller the openings, the greater the flow resistance offered by the filter and the slower the filtration process. The filter may actually remove particles smaller than the filter mesh size due to the buildup of a filter cake. The filter size opening is usually selected to be a compromise between the requirements of metallurgical cleanliness and production efficiency.
Another type of filter used in the aluminum industry for filtering conventional (non-composite) alloys is the porous media filter. The porous media filter is a block of a material such as a ceramic that has a controlled open-cell porosity therethrough. Pieces of undesirable solid material are trapped within the volume of the filter as the molten alloy is passed through the filter.
In the course of the work leading to the present invention, conventional glass-fiber sock filters and porous media filters were used to filter molten mixtures of an aluminum alloy and 10-20 volume percent of desirable particulate such as alumina or silicon carbide, of a size distribution of about 5-20 micrometers. Coarse undesirable solid matter was mixed in to the melt. The conventional filtering practice could be used on a laboratory scale. However, it did not produce successful commercial-scale heats of the composite material. Variations of filter opening size were also tried, unsuccessfully. In short, conventional aluminum-alloy filtering practice was not operable with aluminum-based cast composite materials on a commercial scale.
There is therefore a need for an improved filtering technology for removing undesirable large solid pieces from composite material melts, while not affecting the distribution of smaller particles in the melt and the final product. The present invention fulfills this need, and further provides related advantages.